The F ratio is dependant on the storm and light, same as the exposure time is dependant on the light and storm, beauty of digital photography is you can see your results, dont be afraid of losing a few strikes to get your settings right, you will be rewarded. Use the lightning as the flash, stop the exposure after the bolt frame your pictures and most importantly be aware of your surroundings, nothing worse than someone sneaking up behind you and scarring the living daylights out of you! Also dont hide under trees, some have a habbit of dropping sap, oh and lightning usually hits them too